A D-class signal amplifier of low power consumption (D-class amplifier) is mainly used as a power driving circuit for driving a speaker. Such a D-class signal amplifier is comprised of CMOS inverters connected to each other in multiple stages each made up of n-channel and p-channel MOS transistors, and it is driven by a PWM control. In the case of outputting a PWM waveform by connecting the CMOS inverters to each other in multiple stages, the PWM output signal includes a phase component which is determined by a power center other than an amplitude component which is determined by a pulse width, wherein if the PWM waveform at rise time is different from that at fall time, there is a likelihood of the occurrence of the displacement of the power center, causing a problem that an output signal to the speaker is distorted.
In a BTL system for outputting waveforms which are displaced by 180 degrees in phase between a +side input line of a speaker and a −side input line thereof, noises at overshoot/undershoot of a PWM output signal or at rise time and that at fall time and so forth are cancelled by a stray capacitor between the +side input line and the −side input line, and electromagnetic waves caused by current fluctuation at the +side input line and the −side input line are also cancelled each other. The cancellation of the noises and electromagnetic waves is found that the closer the waveform of the PWM output signal at the rise is relative to the waveform thereof at the fall, the more effect is achieved while the larger the difference between these waveforms is, the less effect is achieved.
The waveform of the PWM output signal at the rise and the same at the fall depend on a driving capacity of the p-channel transistor and the n-channel transistor, and if the driving capacities of the p-channel transistor and n-channel transistor are different from each other, there occurs a difference between the rise and the fall of the PWM output signal. Meanwhile, the driving capacities of the p-channel and n-channel transistors are varied in the fabrication thereof, and hence there occurs a problem that a large difference between the waveform at the rise and that at the fall according to a fabrication lot in a conventional D-class signal amplifier, and also the distortion of waveforms becomes large so as to largely influence upon an EMI (Electro Magnetic Interference).
JP-A 2001-223537 (page 2 to 4, FIGS. 1 to 3) discloses a D-class signal amplifier having a function of adjusting an output waveform to be supplied to a speaker. According to this D-class signal amplifier, the number of selection of the CMOS inverters is increased or decreased, thereby adjusting the output waveform to the speaker.
According to this D-class signal amplifier as disclosed in JP-A 2001-223537, there is a problem that the number of selection is changed by a unit of a transistor pair made up of p-channel and n-channel transistors, thereby increasing or decreasing the number of selection of the p-channel and n-channel transistor at the same time, and in the case where there is a difference between the driving capacity of the p-channel transistor and that of the n-channel transistor, it is difficult to achieve the balance therebetween, so that there occurs a large difference between the output waveform at the rise and that at the fall, and the distortion of the waveforms becomes large to influence upon the EMI.